The present invention relates to locating index marks in a magnetic disk storage system.
In a magnetic storage system, such as a computer disk drive, digital information is magnetically stored upon a surface of a magnetic medium such as a magnetic storage disk. The digital information is represented by selectively polarizing the magnetic field of consecutive areas across the surface of the magnetic disk. When this information is read back from the storage disk, the magnetic polarization of the medium is sensed and converted to an electrical output signal. The read and write operations are performed by a magnetic read/write head which "flies" over the surface of the rotating disk and provides an output signal.
Typically, storage disks of a disk drive are stacked in a "disk pack." Each side of each disk in the pack is used to store information. Each side of each disk in the pack has at least one magnetic head responsible for reading and writing information on that particular surface. Typically, all the magnetic heads move in tandem radially over the surfaces of the disks so that they are all at the same disk radius at the same time. A "cylinder" identifies the set of specific tracks on the disk surfaces in the disk pack which lie at equal radii and are, in general, simultaneously accessed by the collection of heads. Each track within a cylinder is divided into frames or sectors for carrying data. A servo surface occupies one side of one of the disks in the pack and carries information used by a controller to, among other things, generate an index pulse when a specific frame ("the index frame") on the servo surface is detected. The information stored on the servo surface is used to locate the position of magnetic heads over the other disk surfaces in the pack. Data on the servo surface is used to locate sectors on the data carrying surfaces. The sectors are located with reference to the position of index marks on the servo surface.
Typically, prior art index generation methods involve the recognition of a specific non-zero index mark written only in the index frame, with a zero pattern written in all other servo frames.
The prior art index generation circuitry is highly susceptible to readback errors. Errors from a single frame can cause either failure to detect index in the desired frame or false generation of index in an undesired frame.
These schemes are susceptible to both missing the desired index due to a drop out of a bit (reading "zero" instead of "one") or bits in the non-zero index mark, and to falsely identifying an index in an undesired frame due to a drop in (reading "one" instead of "zero") of bits in a pattern identical to the specific non-zero index mark. It would be highly desirable to provide a method and apparatus for generating index pulses which has a high level of error tolerance in the index information read back from the servo surface.